At the beginning of class, the noise of desks _____ could be heard outside the classroom.

A．opened and closed

B．to be opened and closed

C．being opened and closed

D．to open and close

浓缩塔精馏效果不好不是（）引起的。

A.真空度

B.空塔速度

C.回流比

D.工艺水波动